|
Volcanoes are openings in the surface of the earth which molten rock (magma) and gases can escape. The earth is made up of three layers - crest, mantle and core. The crust is the outermost layer of rock and can be quite thin. The crust feels solid but is actually made up of giant plates that float on the upper mantle. The upper mantle is made of magma and is always moving. Pressure in the mantle forces magma to the surface. Volcanic eruptions occur where the rising magma finds a way through a crack or weakness in the earth's crust, usually at the plate margins. Types of Volcano Thick, slow moving andesite lava builds up high, cone-shaped volcanoes. Andesite volcanoes are very violent. Shield volcanoes form when runny lava escapes through a fissure and flows a long way. It has broad and sloping sides like a shield. Parts of a volcano Central vent: Pressure causes gases to build up and force the magma up the central pipe and out through the vent. This creates a deep, steep-walled crater around it. Lava stream: Bubbles of gas force the rising magma to the surface. At the surface, magma is called lava. When it cools, it solidifies into rocks. Magma: Magma collects in a chamber in the upper mantle. It is formed when two plates collide. The edge of one plate moves down under the other and melts into magma. Layers: The steep slopes are built up of alternate layers of ash, and hardened lava. Lava might surface itself through other places and form other cones on the sides of the central cone. Ash: The clouds of ash and gas that pout from the volcano help to form the cone shape around its vent. The ash consists of tiny pieces of lava, which harden in rock called tuff. Back (Volcano main page) | Next (Active or extinct?) |